Structurally reinforced wood particle panels utilizing the shape of the panel itself to provide the reinforcement, are known. A structurally reinforced panel is disclosed in the U.S. Pat. No. 3,083,128 issued Mar. 26, 1963 to Herrington et al. which disclosed a lignocellulose hardboard panel having improved stiffness and dimensional stability and wherein one flat surface is flat or planer and the opposite is formed with a plurality of spaced ribs having a density very significantly lower than the density of the depressed areas (valleys) therebetween. The hardboard panel is produced by pressing a uniform thickness mat so that the depressed areas are squeezed to a thinner, final thickness and thus higher density than the ribbed areas.
Obviously, reinforcing ribs should be as strong as possible if they are to provide a maximum degree of reinforcing. This was recognized by Herrington et al. and in an attempt to provide better reinforcing, Herrington et al operated a press in a manner to produce skins on the surface of the hardboard so that the density adjacent both surfaces were significantly higher than the density in the middle of the board. Obviously, this technique would have limited effect which, in part, would depend on the thickness of hardened surface layer.
The Herrington et al. patent relates to a hardboard formed by the so-called Masonite process wherein the sheet is formed from a specially produced pulp using a wet laying process which lends itself better to deformation and the embossing techniques and the surfacing hardening of the material as described.
Chipboard when it is to be embossed is generally provided with a embossing layer of fine chips as taught, for example, in the U.S. Pat. No. 3,793,125 issued Feb. 19, 1974 to Kunz. In this patent, wafer or chipboard is decorated by applying a layer of fine chips to the surface, overlaying a fine chip layer with a decorative deformable layer and then pressing using a relief carrier.
U.S. Pat. No. 4,061,813 issued Dec. 6, 1977 to Geimer et al. discloses the wood-based building component made of fibers or particles adhesively secured together and shaped in a single pressing to form a flat panel with a plurality of spaced specially shaped channels formed therein.
It has also been taught to form a corrugated shaped board by using a special press on a mat of wood wafers to simultaneously shape and press to consolidate the mat into the corrugated configuration. The press required to do this is relatively complicated (see U.S. Pat. No. 4,616,991 issued Oct. 14, 1986 to Bach).
Of the above described boards only the embossed hardboard of Herrington et al. described in U.S. Pat. No. 3,083,128 has a flat surface on one side and reinforcing rib on the other. However, as the above described, the board of this patent is a hardboard made by wet processing and has significantly different average densities in the skins and center of the panel and in the valley and the rib areas.
U.S. Pat. No. 4,084,996 issued Apr. 18, 1978 to Wheeler describes a plywood panel provided with a plurality of embossed parallel grooves formed therein by a caul plate having a plurality of spaced ridges corresponding to the desired grooves in the panel.
U.S. Pat. No. 4,308,308 describes a ribbed, reinforced fiberous material panel formed, for example, of fiberglass. It is not clear in the patent how the ribs are formed. In any event, the resultant structure is consolidated by heat only without the application of pressure. The patent shows a fiberous product with integral ribs that presumably are of essentially the same density as the valley areas therebetween, however, as above described the product is a cured fiberglass product not a pressed lignocellulose product.
The nonuniform application of pressure when uneven surfaced panels are being consolidated and formed by pressing results in major differences in the densities of various areas of the consolidated product, i.e. the areas pressed the most (thinner areas) have the highest densities while for reinforcing the reverse is more desirable.